Direct-positive print-out silver halide emulsion fogged to visible density

ABSTRACT

Print-out, direct-positive images are obtained by exposure of a silver bromide emulsion which has been uniformly fogged to a visible density and contains an electron-accepting compound.

United States Patent 7 Gilman, Jr.

[ 51 Mar. 21, 1972 DIRECT-POSITIVE PRINT-OUT SILVER HALIDE EMULSION FOGGED TO Appl. No.: 56,703

U.S. Cl ..96/10l ..G03c 1/36 Field of Search ..96/ 101 [56] References Cited UNITED STATES PATENTS 3,501,310 3/1970 lllingsworth et al ..96/l0l X 3,526,507 9/1970 Ishikawa et al. ..96/10l Primary Examiner-Norman G. Torchin Assistant Examiner-Won H. Louie, Jr.

Attorney-W. H. J. Kline, Bernard D. Wiese and Gerald E. BattiSt [57] ABSTRACT Print-out, direct-positive images are obtained by exposure of a silver bromide emulsion which has been uniformly fogged to a visible density and contains an electron-accepting compound.

17 Claims, No Drawings DIRECT-POSITIVE PRINT-OUT SILVER I-IALIDE EMULSION FOGGED TO VISIBLE DENSITY It is well-known that photographic materials for direct image recording are of two types: develop-out as disclosed, for example, in U.S. Pat. No. 3,501,305 and print-out as disclosed, for example, in U.S. Pat. No. 3,189,456. The developout type requires that the exposed material be chemically developed, fixed and washed to provide a stable visible image. The print-out type requires no chemical processing and may or may not be stabilized and fixed. These materials generally have a much slower speed than develop-out materials. Moreover, the images are unstable, as they are obliterated when the background density increases as a function of time. The known print-out materials are of the negative type, i.e., they produce density in the area of exposure.

Accordingly, it is an object of my invention to provide novel photographic materials and methods for forming a direct-positive print-out image on exposure to radiation.

lt is a further object to provide such materials and methods which are capable of forming this direct-positive print-out image at exposure levels normally used to form negative images in print-out materials.

lt'is still a further object to form said direct-positive printout image without the necessity of a developer solution.

In accordance with my invention, the above objects are attained by my novel method and photographic materials. The method, in general, comprises fogging to a visible density, e.g., from 0.30 to 2.0, and preferably from 0.50 to 1.5, a bulk silver halide emulsion wherein at least 50 mole percent of the halide of said silver halide is bromide. An electron-acceptor compound which has a reduction potential less negative than minus 1.0 and an oxidation potential more positive than plus 0.4 is added to the visibly fogged emulsion in the dark, preferably with agitation and while said emulsion is in the bulk state. The resulting emulsion can then be coated on a suitable paper or film support to obtain the novel photographic element of the invention comprising a support coated with a silver halide emulsion of which the silver halide content is at least 50 mole percent bromide, the emulsion being visible fogged prior to image exposure to a density of at least 0.3 and containing an electron-acceptor compound which has a reduction potential less negative than minus 1.0 and an oxidation potential more positive than plus 0.4.

l use the expression visible density to distinguish from developable latent density which can exist, for example, in a silver halide emulsion that has been fogged less than the emulsions employed in the present invention. When I say that the bulk emulsion has a visible density of at least 0.3," I mean that its density is such that, when coated on white paper to the desired thickness, the visible density of the coating without development and measured by reflection'densitometry will be at least 0.3. This means, of course, that when the emulsion is to be employed as a thick coating, less bulk fogging exposure will be required to produce the visible density of at least 0.3 than when a thinner coating is employed. 1f the print-out image is not subjected to any development step (the possibility of which is discussed hereinafter), the particular density to which the bulk emulsion is fogged will be the maximum density of the print-out image.

The silver halide emulsions which can be fogged and used in combination with the electron acceptors according to this invention to provide direct-positive print-out emulsions generally include silver bromide, silver bromoiodide, silver chlorobromoiodide and the like or mixtures thereof. However, the halide of the silver halide is at least 50 mole percent bromide. For certain highly advantageous embodiments, the halide of the silver halide contains less than mole percent iodide. The emulsions may be coarse-grain or fine-grain emulsions and can be prepared by any of the procedures used for making silver halide emulsions, e.g., single-jet procedures, double-jet procedures, procedures using ammoniacal, thiocyanate and/or thioether ripening agents and the like.

The fogging of the silver halide to a visible density of at least 0.3 is accomplished by light fogging the bulk silver halide emulsion while agitating it. Light fogging" is used herein in its broad sense to include exposure to any visible or invisible electromagnetic radiation that will fog the silver halide. This will normally be radiation in the range of intrinsic sensitivity of silver halides which is in the ultraviolet to blue range of the spectrum. It is desirable to employ a primitive silver halide emulsion during the fogging stage of preparation, i.e., one which contains no addenda such as spectral-sensitizing dyes. However, it is also within the scope of the invention to employ emulsions which contain addenda which do not adversely affect the ultimate production of a direct-positive print-out image.

As mentioned, the silver halide is agitated during the fogging exposure. The reason for this is to achieve uniform fogging of the silver halide grains. Any of the normal means for stirring and agitating silver halide emulsions during formulation can be employed.

Various combinations of intensity of illumination and duration of exposure can be employed which will achieve the desired visible density and uniformity of fogging. Thus, since exposure is equal to intensity multiplied by time, one can employ a high intensity for a relatively short time or a lower intensity for a longer time.

Various sources of the desired radiation can be employed, including daylight, but normally it is more convenient to use some type of artificial illumination such as incandescent tungsten lamps arranged to give maximum exposure, e.g., including use of a transparent mixing vessel for the emulsion.

Light fogging offers important advantages in achieving the desired visible density in the fogging stage of my method. For instance, the fog produced by light fogging of the grains bleaches readily in the subsequent imagewise exposure of the photographic elements in the presence of electron acceptors.

Although the fogging step preferably consists of radiation exposure of about 300 to 800 nm., the direct-positive printout emulsions of this invention can be made in certain embodiments with a combination of light fogging with chemical fogging, and the expression light fogging" is used herein to include light fogging alone or a combination of light and chemical fogging. In this event, the silver halide can be chemically fogged, for example, with a reducing agent such as thiourea dioxide, hydrazine compounds or stannous chloride or with a combination of such a reducing agent and a compound of a noble metal such as gold, as disclosed, for example, in British Pat. No. 723,019. Advantageously, the bulk silver halide, as in light fogging, is agitated during such reduction in order to insure uniformity of visible fogging.

The electron-acceptor compounds useful herein are organic compounds which can be termed desensitizing electron acceptors and have been referred to in the art as desensitizers for negative silver halide emulsions. They can be characterized by their polarographic half wave potentials, i.e., their oxidationreduction potentials as determined by polarography. Thus, they are organic compounds having a reduction potential less negative thari'minus 1.0 and an oxidation potential more positive than plus 0.4.

To determine the oxidation and reduction potentials of such compounds, cathodic measurements can be made with a l X 10 molar solution of the electron acceptor in a solvent, for example, methanol which is 0.05 molar in lithium chloride using a dropping mercury electrode, with the polarographic half wave potential for the most positive cathodic wave being measured. Anodic measurements can be made with l X 10 molar aqueous solvent solutions, for example, methanolic solutions of the electron acceptor which are 0.05 molar in sodium acetate and 0.005 molar in acetic acid using a carbon paste of pyrolytic graphite electrode, with the voltametric half peak potential for the most negative anodic response being measured. In each measurement, the reference electrode can be an aqueous silver chloride (saturated potassium chloride) electrode at 20 C. Electrochemical measurements of this type are known in the art and are described, for example, in New instrumental Methods in Eiectrochemistry, by Delahay, Interscience Publishers, New York, N.Y., 1954; Polarography, by Kolthoff et al., 2nd Edit. lnterscience Publishers, New York, N.Y., 1952; Anal. Chem., 36, 2,426 (1964), by Elving; and Anal. Chem., 30, 1,576 (1958), by Adams. Plus and minus signs are according to lUPAC, Stockholm Convention, 1953.

The significance of these limits for the oxidation-reduction potentials of the electron-acceptor compounds is that if the oxidation potential is less positive than plus 0.4, the compound will not be capable of oxidizing and thereby bleaching surface fog of the silver halide upon imagewise exposure. If the reduction potential is more negative than minus 1.0, the compound will increase rather than decrease the density of the prefogged emulsion, i.e., by photoreduction of silver halide.

The electron-accepting compounds or desensitizers are used in the emulsions of this invention in a desensitizing concentration, i.e., a concentration which would provide at least a 0.3 log E decrease in blue-speed sensitivity ofa sulfur and gold surface-sensitized silver bromoiodide (6 mole percent iodide) emulsion when developed at 25 C. in a surface developer such as Kodak D-19.

The electron-accepting desensitizing compounds can be selected from a wide range of such compounds which have the required oxidation and reduction potentials, including various photoelectron-accepting compounds or desensitizing dyes used in photographic reversal systems. Compounds of this type include the known desensitizers which trap electrons as disclosed in British Pat. No. 723,019. Also included are spectral-sensitizing electron acceptors such as disclosed in U.S. Pat. Nos. 3,501,305 and 3,501,310. These include various cyanine dyes, particularly imidazoquinoxaline dyes such as those disclosed in Brooker et al., Belgian Pat. No. 660,253 of Mar. 15, 1965; the bis-( l-alkyl-2-phenylindole-3) trimethine cyanines described by Coenen et al., U.S. Pat. No. 2,930,694 issued Mar. 29, 1960; and the dimethine dyes disclosed in British Pat. No. 970,601. Still another suitable class of electron-acceptor compounds from which those of the required oxidation and reduction potentials can be selected include bipyridinium salts which have two pyridinium nucleijoined by a linkage attached to a carbon atom of each of the nuclei. The two pyridinium nuclei can be joined by any suitable linkage such as a single covalent bond, a vinylene linkage or an alkylene linkage. Such bipyridinium compounds are disclosed in Gilman, US. Pat. application, Ser. No. 776,262 filed Nov. 15, 1968; Fry et al., U.S. Pat. No. 3,035,917; Fry et al., U.S. Pat. No. 3,124,458; British Pat. No. 714,290; Belgian Pat. No. 626,303; and Homer et al., J. Chem. Soc., 1960, 2,498.

Examples of electron-acceptor compounds having oxidation and reduction potentials within the indicated limits and which are particularly useful in accordance with my invention include the two well-known desensitizing dyes phenosafranine and pinakryptol green which can be represented as follows:

phenosafranine N Oxidation potential: +1.0 Reduction potential: 0.64

mN- g NH; 01

pinaknitol g reen Hz Oxadation potential: +1.0 N Reduction potential: --0.66 NH, 01

Further examples of suitable desensitizing electron acceptors are as follows (their oxidation and reduction potentials are listed at the end ofthe list offormulae):

ABis(l, 2-dimethy1-3-indole) trlmethine cyanine iodide T CH=CHC H J C C H3:L$ N N on, em 1 B-l, 1-dimethyl-2, 2-diphenyl-3. 3-ind01ocarbocyanine bromide CH-N i i on CH on C1,1,3,3-tetraethyllmidazo{4,6-b]quinoxalinocarbocyanine chloride E5-chloro-1,3-dimethyl-2-phenyl-6'-nitro-3-indol0thiacarboeyanine pts N02 om- CH=CHC cm vms e G-2,2diethyl-6,6'-dinltrothioearbocyanlne chloride H-2-[3-(2-phenyl-3-indo1yl)allylidene]-2-phenyi-3H-indole Ph Ph I-l,1'-9-triethy1-2,2-cyanlne perchlorate i we J 2,3,2,3'-tetrapheny1-1,1-pyrrocoloearbocyanine bromide Ph Ph K'-9-(Z-carboxy-G-nitrophenyl)-3,3-dtmethylthiaoarb0cyanme pts M pts Ii1,1',3,3-tetraethyl1mldazo [4,5-b] qulnoxalinodlcarbooyanine chr e N-1-(p-dimethylamlnoclnnamylidene)-2,3-diphenylpyrrocolinlum perchlorate CHCH=CH-NMez 03-(p-dimethylaminocinnamylidene)-1,2-diphenylpyrrocolmium iodine P 1,3-diethyl-1-methyl-2'-phenylimidazo[4,5-b] quinoxal1110-3-indolocarbocyanine iodide Me-N Q-1,3-dial1yl-2-[2-(3, 5-dimethy1-1-phenyl-4-pyrazolyl) vinyl]-imidazo[4,5-b]quin0xalinium iodide allyl 0 l R6,6,7,7-tetrachloro-1,-1,3,3-tetraphenylimidazo [4,5-b]-quinoxa.linocarbocyanine pts Dyc Oxidation Potential Reduction Potential A +l .0 0.31 a +l .0 4.20 C +1 .0 4.79 D +l .0 0,60

E l .0 0.42 F +0.95 0.8l

G +0.90 0.60 H +0.73 -0.20 l +0.77 -0.86 1 +0.76 -0.33 K +0.66 0.31

L +0.66 0.64 M +0.52 0.60 N +0.5 l 0.21 0 +0.44 O. 18

P +l .0 0.64 Q l .0 --0,45 R l .0 0.45

The electron-acceptor compound is incorporated in the emulsion in a desensitizing amount, that is to say, an amount efficient to prevent latent negative image formation during the image-exposure step. The electron acceptor is used to desensitize the emulsion and, therefore, enough is added so that a negative image is not formed when the prefogged emulsion is exposed imagewise. The specific concentration which gives best results for each combination of variables, e.g., the particular emulsion and desensitizing compound, can be determined by routine testing, but will normally lie in the range from about 10 to 1000 mg. per mole of silver halide.

The electron acceptor is added to the silver halide emulsion in a desensitizing amount by any of the known emulsion formulating techniques. Thereafter, the emulsion is coated on a support to form a photographic element at a coverage, for example, of about 50 to 500 mg. of silver per square foot of support.

The following examples of the preparation of photographic elements provide a further understanding of this invention. Example 1:

To 100 ml. of a 5 percent gelatin solution are added 25 g. of a silver bromide emulsion (1.72 kg. of emulsion contains 1 mole of silver and 80 g. of gel). This melted emulsion, while being agitated, is then exposed to a l00-watt tungsten bulb for 10 minutes at a distance of 12 inches, at which time it reaches a visible density when coated of about 0.5.

In the dark are added 1.0 ml. of 7%percent saponin solution and 0.5 ml. of IOpercent formaldehyde. To separate l0 ml. portions of the above mixture are added, in the dark, 0.5, 1.0 and 2.0 ml. of a 0.1percent by weight aqueous solution of phenosafranine. The emulsion is coated to a wet thickness of 0.004 inch of a reflection print support.

When the dried coatings which appear dark (density 0.5) are exposed to a No. 2 photoflood lamp at a distance of 24 inches, direct-positive printout images are obtained with all three coatings, i.e., in the areas of exposure, the fogged silver is bleached by the action of the light. The density range of the prints is 0.5 Dmax and 0.05 Dmin. The coating is rinsed with water and dried to produce a stable image.

Print-out wedge spectrograms made on the coating containing 1 milliliter of phenosafranine dye solution to ml. of emulsion show two peaks, one from 350 to 450 m. and another which extends from 550 to 620 m., indicating that the silver is being bleached by the action of light absorbed both by the silver bromide and the phenosafranine dye.

Example 2:

When the dye pinakryptol green is substituted for the phenosafranine in the coating formulation described in Example 1, similar direct-positive print-out characteristics are obtained.

From the foregoing examples, it can be seen that a directpositive print-out element with satisfactory speed and contrast is provided. They demonstrate an important advantage of the novel photographic elements of providing a direct-positive printout image having adequate stability for many purposes without the necessity of chemical treatment after image-wise exposure. It should be understood, however, that if the electron acceptor is not removed from the element after exposure to an image, the maximum density of the image will gradually decrease when exposed to light or other radiation. Therefore, it is sometimes desirable, in order to stabilize the image, to treat the print-out element in various ways. One suitable treatment is simply to rinse the print-out element with water to remove the electron-acceptor compound or compounds. This provides improved stability, but for maximum stability it is also possible to fix the image before or after rinsing by treatment with a solution of sodium thiosulfate or other fixing agent. Furthermore, it is also possible to treat the element with a silver halide developing agent to increase the maximum den-- "of much higher intensity such as exposure to a laser beam,

e.g., as disclosed in U.S. Pat. No. 3,474,457 or to an electron beam as disclosed in U.S. Pat. No. 3,510,348.

1 do not wish to be bound by any theoretical explanation of the mechanism of the formation of positive print-out images in the process of my invention. It is believed, however, that the desensitizing property of the electron-acceptor compounds is the result of a regenerative act. It appears that the desensitizers which act well in the process are those which are capable of competing for a photoproduced electrons with the latent image traps existing in the emulsion. For a desensitizer of this type to be useful in a direct-positive print-out system, it must be capable of reuse and thus must not only be an electron acceptor but must also get rid of the electron to its environment so that the desensitizing cycle may continue for a duration of the print-out exposure. Since a positive hole is produced along with an electron at the time of exposure, a proposed mechanism is that the process depends on the continued attack by positive holes for the preformed print-out silver until suflicient degradation takes place to form a visible difference between exposed and unexposed areas.

The photographic layers used in the practice of this invention may be coated by various coating procedures including dip coating, air knife coating, curtain coating or extrusion coating using hoppers of the type described in Beguin, U.S. Pat. No. 2,681,294. If desired, two or more layers may be coated simultaneously by the procedures described in Russell, U.S. Pat. No. 2,761,791, and Wynn, British Pat. No. 837,095. This invention also can be used for silver halide layers coated by vacuum evaporation as described in British Pat. No. 968,453 and LuValle et al., U.S. Pat. No. 3,219,451.

The photographic layers and other layers of a photographic element employed as described herein can be coated on a wide variety of supports. Typical supports include cellulose nitrate film, cellulose ester film, poly(vinyl acetal) film, polystyrene film, poly(ethylene terephthalate) film, polycarbonate film and related films or resinuous materials, as well as glass, paper, metal and the like. Typically, a flexible support is employed, especially a paper support, which can be partially acetylated or coated with baryta and/or an alpha-olefin polymer, particularly a polymer of an alpha-olefin containing two to 10 carbon atoms such as polyethylene, polypropylene, ethylenebutene copolymers and the like.

The silver halide emulsions used with this invention may be unwashed or washed to remove soluble salts. 1n the latter case, the soluble salts may be removed by chill-setting and leaching or the emulsion may be coagulation-washed, e.g., by the procedures described in U.S. Pat. No. 2,618,556 by Hewitson et al., 2,614,928 by Yutzy et al., 2,565,418 by Yackel, 3,241,969 by Hart and 2,489,341 by Waller et al.

The photographic and other hardenable layers used in the practice of this invention can be hardened by various organic or inorganic hardeners, alone or in combination, such as the aldehydes, and blocked aldehydes, ketones, carboxylic and carbonic acid derivatives, sulfonate esters, sulfonyl halides and vinyl sulfonyl ethers, active halogen compounds, epoxy compounds, aziridines, active olefins, isocyanates, carbodiimides, mixed-function hardeners and polymeric hardeners such as oxidized polysaccharides, e.g., dialdehyde starch, oxyguar gum and the like.

The photographic emulsions and elements can contain various colloids alone or in combination as vehicles, binding agents and various layers. Suitable hydrophilic materials include both naturally occurring substances such as proteins, for example, gelatin, gelatin derivatives, cellulose derivatives, polysaccharides such as dextran, gum arabic and the like; and synthetic polymeric substances such as water-soluble polyvinyl compounds like poly(vinylpyrrolidone), acrylamide polymers and the like.

The described photographic emulsion layers and other layers of a photographic element employed in the practice of this invention can also contain, alone or in combination with hydrophilic, water-permeable colloids, other synthetic polymeric compounds such as dispersed vinyl compounds such as in latex form and particularly those which increase the dimensional stability of the photographic materials. Suitable synthetic polymers include those described, for example, in U.S. Pat. Nos. 3,142,568 by Nottorf issued July 28, 1964; 3,193,386 by White issued July 6, 1965; 3,062,674 by Houck et a1. issued Nov. 6, 1962; 3,220,844 by Houck et al. issued Nov. 30, 1965; 3,287,289 by Ream et a1. issued Nov. 22, 1966; and 3,411,911 by Dykstra issued Nov. 19, 1968; particularly effective are those water-insoluble polymers of alkyl acrylates and methacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates, those which have cross-linking sites which facilitate hardening or curing, and those having recurring sulfobetaine units as described in Canadian Pat. No. 774,054 by Dykstra.

The photographic elements used with this invention may contain antistatic or conducting layers; such layers may comprise soluble salts, e.g., chlorides, nitrates, etc., evaporated metal layers, ionic polymers such as those described in U.S. Pat. No. 2,861,056 by Minsk and 3,206,312 by Sterman et al. or insoluble inorganic salts such as those described in U.S. Pat. No. 3,428,451 by Trevoy.

The photographic layers employed in the practice of this invention can contain plasticizers and lubricants such as polyalcohols, e.g., glycerin and diols of the type described in Milton et al., U.S. Pat. No. 2,960,404; fatty acids or esters such as those described in Robijns, U.S. Pat. No. 2,588,675 and Duane, U.S. Pat. No. 3,121,060; and silicone resins such as those described in DuPont, British Pat. No. 955,061.

The photographic layers employed in the practice of this invention may contain surfactants such as saponin; anionic compounds such as alkyl aryl sulfonates described in U.S. Pat. No. 2,600,831 by Baldsiefen; amphoteric compounds such as those described in U.S. Pat. No. 3,133,816 by Ben-Ezra; and

invention may contain matting agents such as starch, titanium dioxide, zinc oxide, silica, and polymeric beads including beads of the type described in US. Pat. Nos. 2,992,101 by Jelley et a1. and 2,701,245 by Lynn.

The dyes and other addenda used in the practice of this invention may be added from water solutions or suitable organic solvent solutions may be used. The compounds can be added using various procedures including those described in US. Pat. Nos. 2,912,343 by Collins et a1.; 3,343,605 by McCrossen et al.; 2,996,287 by Audran; and 3,425,835 by Johnson et al.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

1 claim: 1. A method of manufacture of a photographic element useful as a direct-positive printout element which comprises:

a. light fogging to a visible density of at least about 0.3 a bulk silver halide emulsion of which the halide is at least 50 mole percent bromide,

b. adding to said fogged silver halide emulsion in the dark a desensitizing amount of a desensitizing electron-acceptor compound which has a reduction potential less negative than minus 1.0 and an oxidation potential more positive than plus 0.4 and c. coating the silver halide emulsion on a support in the darkv before or after the addition of said electron-acceptor compound.

2. A method according to claim 1 in which the bulk silver halide emulsion is agitated while being light fogged to a density of0.3 to 2.0.

3. A method according to claim 1 in which the halide content of the emulsion is less than about mole percent iodide and in which the emulsion is light fogged to a density from about 0.5 to about 1.5 before addition of the electron-acceptor compound.

4. A method corresponding to claim 2 in which the elec tron-acceptor acompound is phenosafranine or pinakryptol green.

5. A methodaccording to claim 2 in which the light fogging of the emulsion includes chemical fogging to achieve a visible density of 0.5 to 1.5 before addition of the electron-acceptor compound.

6. A photographic silver halide emulsion of which the halide content is at least 50 mole percent bromide and which is uniformly light fogged to a visible density of at least about 0.3 and contains a desensitizing amount of a desensitizing electron-acceptor compound, said compound having a reduction potential less negative than minus 1.0 and an oxidation potential more positive than plus 0.4.

7. An emulsion according to claim 6 in which the silver halide is silver bromide or silver bromoiodide containing less than about 10 mole percent iodide.

8. An emulsion according to claim 6 in which said electronacceptor compound is phenosafranine or pinakryptol green.

9. Am emulsion according to claim 8 which is light fogged to a visible density of about 0.5 to about 1.5 before addition of the electron-acceptor compound. 7 7 g 10. An emulsion according to claim 9 in which the halide content is less than 10 mole percent iodide.

11. An emulsion according to claim 10 in which the electron-acceptor compound is phenosafranine or pinakryptol green.

12. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim 6.

13. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim 7.

14. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim 8.

15. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim 9.

16. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim l0.

17. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim 1 l. 

2. A method according to claim 1 in which the bulk silver halide emulsion is agitated while being light fogged to a density of 0.3 to 2.0.
 3. A method according to claim 1 in which the halide content of the emulsion is less than about 10 mole percent iodide and in which the emulsion is light fogged to a density from about 0.5 to about 1.5 before addition of the electron-acceptor compound.
 4. A method corresponding to claim 2 in which the electron-acceptor acompound is phenosafranine or pinakryptol green.
 5. A method according to claim 2 in which the light fogging of the emulsion includes chemical fogging to achieve a visible density of 0.5 to 1.5 before additioN of the electron-acceptor compound.
 6. A photographic silver halide emulsion of which the halide content is at least 50 mole percent bromide and which is uniformly light fogged to a visible density of at least about 0.3 and contains a desensitizing amount of a desensitizing electron-acceptor compound, said compound having a reduction potential less negative than minus 1.0 and an oxidation potential more positive than plus 0.4.
 7. An emulsion according to claim 6 in which the silver halide is silver bromide or silver bromoiodide containing less than about 10 mole percent iodide.
 8. An emulsion according to claim 6 in which said electron-acceptor compound is phenosafranine or pinakryptol green.
 9. Am emulsion according to claim 8 which is light fogged to a visible density of about 0.5 to about 1.5 before addition of the electron-acceptor compound.
 10. An emulsion according to claim 9 in which the halide content is less than 10 mole percent iodide.
 11. An emulsion according to claim 10 in which the electron-acceptor compound is phenosafranine or pinakryptol green.
 12. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim
 6. 13. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim
 7. 14. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim
 8. 15. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim
 9. 16. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim
 10. 17. A photographic element useful as a direct-positive printout element which comprises a support and an emulsion according to claim
 11. 